FIG. 1 was reproduced from a catalog published by Aerzener Maschinen Fabrik GmbH ("Aerzener"), a German manufacturer of rotary piston blowers. FIG. 1 depicts a longitudinal sectional view of a prior art rotary piston blower 10. Two pistons 15 and 20 are synchronized to rotate at the same speed by timing gears 25 and 30 which are mounted on the respective piston shafts 35 and 40. Piston 15 is known as the driven piston because it is mounted on shaft 35 which extends from the housing for coupling to a motor (not shown). Piston 20 is known as a secondary piston because the rotation of piston 20 is governed by timing gears 25 and 30.
FIG. 2 was also reproduced from a catalog published by Aerzener. As shown in FIG. 2, the rotating pistons are shaped so that pistons 15 and 20 are in close proximity with each other and the piston housing without touching either one.
The piston housing has a piston cylinder 45 which has an interior surface in the general shape of two intersecting bore holes. The piston housing comprises two side plates 50 and 55 which have substantially flat interior surfaces which face the ends of the piston.
Diagrams (a) through (d) in FIG. 2 depict a cross sectional view of the inside of the piston housing looking in the direction of piston shafts 135 and 140. Pistons 115 and 120 typically comprise a body with at least two lobes as shown in FIG. 2. The contoured surfaces of lobed pistons 115 and 120 are shaped such that a contoured surface of rotating piston 115 is normally in close proximity with a contoured surface of rotating piston 120. Timing gears (not shown in FIG. 2) prevent pistons 115 and 120 from touching by ensuring that piston shafts 135 and 140 and corresponding pistons 115 and 120 all rotate at the same speed.
FIG. 2 is a series of drawings which diagrammatically depict how a fluid is blown through a rotary piston blower by showing how the pistons cooperate with one another while rotating at the same speed to compress the fluid against the piston cylinder at different angles of rotation. The process fluid enters through an inlet port 160 in piston cylinder 145 while process fluid exits through an outlet port 165 which is typically opposite inlet port 160. Pistons 115 and 120 are shaped so that the inlet fluid stream is substantially sealed from the outlet fluid stream. However, the inlet and outlet fluid streams are never completely sealed because there is a small gap between the two pistons and between the pistons and piston cylinder 145, since these components are preferably not in contact with each other. An advantage of this arrangement is that because there is no contact between the moving internal components no lubricant is required and a rotary piston blower can be used to supply oil-free fluid streams.
With reference again to FIG. 1, the piston housing of conventional rotary piston blowers comprises at least three pieces, namely piston cylinder 45, and side plates 50 and 55. In some cases, particularly for larger blowers, for ease of assembly and maintenance, piston cylinder 45 and side plates 50 and 55 may each comprise more than one piece. To provide better sealing between the piston housing components, gaskets may be used. Any known fastening devices may be used to join piston cylinder 45 with side plates 50 and 55, such as, for example, flanges with bolts and nuts or tapped holes.
Lubricated fixed bearings 70 mounted on side plate 50 support one end of shafts 35 and 40 between the piston housing and timing gears 25 and 30. The opposite end of shaft 40 is supported by floating bearing 75 which is mounted on side plate 55. A fourth shaft bearing assembly 80, which may be, for example a cylindrical roller bearing, supports shaft 35 where it extends from the housing for connection to the motor coupling. Seals 85 are provided between shaft bearings 70, 75 and 80 and side plates 50 and 55 to prevent lubricants from entering the piston housing while also preventing the fluid inside the piston housing from contaminating or blowing lubricants out of the bearings. An additional shaft seal 90 may also be provided where driven shaft 35 extends from the housing.
There are several disadvantages of the prior art rotary piston blowers which are inherent in the typical designs, one of which has been described above and illustrated in FIG. 1. For example, one disadvantage is that the piston housing is typically made from at least three pieces, namely piston cylinder 45 and side plates 50 and 55; all three of these components contribute to the proper alignment of the piston and shaft assemblies so each of these three pieces must be carefully fabricated to ensure proper alignment. In particular, side plates 50 and 55 both provide supports for bearings 70, 75 and 80 so side plates 50 and 55 both need to be accurately machined and carefully assembled with piston cylinder 45 so that piston shafts 35 and 40 and pistons 15 and 20 are properly aligned within piston cylinder 45. Misalignment of piston shafts 35 and 40 can result in unbalanced rotation and/or accelerated wear of the bearings and seals. Severe misalignment can also cause serious damage to pistons 15 and 20 and the housing if the pistons touch each other or the piston cylinder while rotating.
Another disadvantage of prior art rotary piston blowers is that since piston shafts 35 and 40 are supported on both sides of respective pistons 15 and 20, there are four shaft bearings which each require a seal to isolate the interior of the piston housing from the lubricated shaft bearings and timing gears. Known rotary piston blower designs such as the one shown in FIG. 1 must have shafts which extend from both sides of the pistons because the timing gears are located on one side of the pistons and the shaft extends from the other side of the pistons for coupling to a motor. Because rotary piston blowers are commonly used for process streams which are oil-free and sealed against contaminants, it would be beneficial to reduce the number of openings in the piston housing, and the number of seal and bearing arrangements adjacent the piston housing.
In the prior art, it is not known to use a rotary piston blower to supply an oxidant stream to a fuel cell. A rotary piston blower required for supplying an oxidant stream to a portable or low power fuel cell would generally (depending upon the power output of the fuel cell) be smaller than rotary piston blowers which are commonly available. For example, the rotary piston blowers commercially available from Aerzener have intake flow volumes between 30 cubic meters per hour (500 liters/minute) for their smallest capacity model, up to 15,000 cubic meters per hour for their largest capacity model. The flow rate required for a typical portable or low power fuel cell is in the range of approximately 6 cubic meters per hour (100 liters/minute) or less.